This invention relates to a high speed tool steel produced by powder metallurgy, which steel is employed for parts subject to heavy wear, particularly tools. The high speed tool steel contains C, Cr, V, W and/or Mo, possibly contains one or more of Co, Mn, Si or Al, and further contains elements which accompany iron, e.g. P, S, and O, the remainder of the composition being iron and impurities.
Such high speed tool steels are used, inter alia, as materials for manufacturing tools of the cutting type for machining applications, e.g. milling cutters, drill bits, reaming bits, or broaches; or tools of the non-cutting fabrication type, e.g. drawing dies, extrusion molding plungers, etc.
In the production of high speed tool steels alloyed with Nb, using molten metallurgical techniques, very large inclusions of niobium carbides of type MC may occur, which have grain sizes of over 100 micron. These are detrimental to the impact strength and cutting-edge wear resistance of parts subject to heavy wear fabricated from such high speed tool steels. Further, Nb is only slightly soluble in the base material of the alloys. Therefore, high speed tool steels alloyed solely with Nb generally do not have good secondary hardness properties.
The alloying element V also forms carbides of type MC. However, these have lower thermal stability than Nb carbides. Accordingly, when high hardening or austenitizing temperatures are employed, such as are required in manufacturing cutting tools in order to achieve the desired working properties, viz. hardness, the austenitic grains are undesirably increased in size, as are the deposited carbides, which results in reduced impact strength.
In attempts to alloy high speed tool steel with Nb, higher Nb contents, e.g. &gt;1.5%, led to formation of large-grained Nb carbides, with degradation of the impact strength properties of the resulting tools, and breakingoff of parts of the cutting edges during use. A powder-metallurgical method of producing high speed tool steel is disclosed in Japanese Patent No. 144456/83, wherein the Nb concentration in the steel is limited to 0.1-1.5%, and it is represented that high W and/or Mo contents result in improved hardness following heat treatment.
The object of the instant invention is to devise high speed tool steels having thermal stability in addition to adequate wear resistance and hardness. Further, the steels should have a uniform fine distribution of carbides, in order to yield good impact strength properties, particularly at the locations of sharp cutting edges. In addition, hardness values up to 70 HRC should be attainable.